Conventionally, to securely transmit image data or the like, the entire image data is encrypted or scrambled. In this technique, whole image data is encrypted in advance by using an encryption key. Only a person who has a decryption key corresponding to the encryption key can correctly decrypt the image data.
However, for image data having a hierarchical structure, it is demanded that not the entire image data but the respective layers of the hierarchical structure be encrypted using different encryption keys for the purpose of controlling the reproduction of each layer of the image data. For image data constructed by a plurality of tiles, it is demanded that the respective tiles be encrypted using different encryption keys in order to control the reproduction of each tile. For a combination of image data of these types, i.e., when image data is constituted by a plurality of tiles, and each tile has a hierarchical structure, encryption processing is executed by using different encryption keys for the respective layers in the tiles aiming at controlling the reproduction of each tile and layer of the image data.
In this way, when the respective tiles and layers are encrypted using different encryption keys, the reproduction of image data can be controlled for each tile and layer.
However, to decrypt a predetermined tile or layer of encrypted image data, it is necessary to manage all encryption keys used for encryption processing and supply an appropriate decryption key for decryption processing.
When the respective tiles and layers are encrypted by using different encryption keys, the encrypted tiles and layers must be made to correspond to decryption keys to be used to decrypt them. This inevitably complicates key information management.
If the key information is not properly managed, it is impossible to correctly decrypt a tile or layer.